Since the development of the wind scoop on tractor trailer trucks, such scoop appearing on top of the tractor to deflect the wind resistance of the trailer, what has occurred is that in rainy, slushy or snowy weather, the water or slush or snow, along with salt from the road, runs down off the wind scoop across the top of the cab along the connecting bars of the side-mounted rear view mirrors and then runs down and across the face of the mirrors themselves so that the driver has no rear vision whatever.
The rear-view mirrors on tractor-trailer cabs may be, for example, and often are, 161/2 inches long and 7 inches wide. There is a specific point on the inside of these mirrors, which I will call the weather margin, where precipitation and road film tend to land. Water never runs from the top down unless the truck is completely stopped. Nor does it come from the outside of these mirrors. It always comes from the inside because of the way the mirrors are angled so that the driver can have proper vision.
The top of my device, for the example cited, covers 91/2 inches over the top of these mirrors, coming just below the curve in the glass, with an inside shield of 12 inches to prevent bad weather from impairing the vision of a driver. The visor that covers the top of these mirrors is 41/4 inches deep. My shield is a continuous piece stretching for 211/2 inches. It was not designed to dress up or improve the looks of these mirrors, but to do a job which has been needed for years. It does it efficiently and economically.